Both physics and radiobiology provide growing points in modern radiotherapy. Better physical dose distributions appear to be still worth achieving and can be obtained from beams of protons, heavy ions, or negative pi mesons because a peak region of high dose is deposited at depth in tissue. The heavier ions and pions also have biological properties of high LET radiation which could be important: the radioresistance of hypoxic cells in tumors is less, and tissues which are proliferating fast may be relatively more vulnerable. Although fast neutrons provide ordinary physical dose distributions, their high LET properties are similar to those of ions as heavy as neon. Drugs which specifically radiosensitize hypoxic cells offer a way of determining with certainty how important hypoxic cells are in radiotherapy. Hyperthermia is in its early stages but promises to damage just those cells poor in nutrients which are relatively resistant to ionizing radiation. Radioprotecting drugs, which depend upon poor uptake in tumors but high uptake in normal tissues, are also being tested.

Research for the application of NMR principles to the noninvasive measurement of blood flow in humans began at the National Heart,Lung, and Blood Institute (NHLBI) in 1956, and has continued to the present at a number of institutions. In addition to NHLBI, contributions for the development of blood flowmeters by the University of California, Berkeley and by the Medical College of Wisconsin are described in this peper. The NMR theory applicable to blood flowmeters is also presented, as well as design criteria of NMR blood flowmeters.

Two types of crossed‐coil nuclear magnetic resonance(NMR) blood flowmeter detectors have been developed for the noninvasive measurement of blood flow. The first is a cylindrical coil configuration suitable for limb blood measurement. A cylindrical flowmeter (12.5 cm internal diam) operating at a nuclear resonance frequency of 3.2 MHz has been applied to measurement of flow in the forearm. The second type is the flat crossed‐coil detector, which retains many of the operational advantages of the cylindrical detector, but is suitable for blood flow measurement of almost any surface of the body. Three flat crossed‐coil detectors are described, operating at NMR frequencies of 9, 21.4, and 75 MHz. Two types of intermediate frequency signal processors have been used in the NMR receivers, a simple diode type, and a synchronous detector. The synchronous detector is preferred for its ease of operation and superior stability. Modular detection systems containing transmitter, receiver, post‐detector signal conditioning, and power supply have been designed for all of the flat crossed‐coil detectors. A self‐contained synchronous detector module is included in the 21.4 and 75 MHz systems.

Nuclear Magnetic Resonance(NMR) permits the noninvasive measurement of blood flow signals unimpaired by clothes, bandages, casts, etc. The cylindrical crossed‐coil NMR blood flowmeter was used to measureblood flow through a cross‐section of the human forearm. Two calibration procedures are described: one for pulsatile flows and the other for flows with a high non‐pulsatile component. Flow measurements from normal arms, from limbs with arterial obstruction, arteriovenous hemodialysis fistulas or other conditions are reported. An application of the flow scanning technique for separation of flow signals from individual arteries (e.g., ulnar and radial) is described. The flat crossed‐coil NMR flowmeter was applied to detect blood flow from individual arteries (e.g., brachial, popliteal, etc.). Applications of a ranging technique developed to detect signals at predetermined depths are described.

The shape of the surface of incidence could affect the scatter and change the effective tissue–air ratio (TAR) value. The curvature of the body is considerable within the large beams used for total body irradiation. The effect of the surface curvature on the effective TAR value was studied using a computational approach that sums up the differential scatter air ratios from elemental beams. It was observed that the difference between the effective TAR and the standard flat surface TAR increased with depth to reach a maximum value of about 2% around 8–11 cm depth. It decreased for larger depths, and the effect of surface shape was not perceivable beyond 18 cm depth.

Two new types of thermographic instruments sensitive to millimeter‐wave electromagnetic radiation have been designed, constructed, and tested. These instruments utilize wavelengths that are three orders of magnitude longer and much more penetrating then those used in conventional infrared thermography. The instruments are capable of detecting apparent thermal variations as small as a fraction of a degree existing at tissue depths of several millimeters below the skin. By comparison, conventional IR thermographic units are limited to sampling radiation emitted only from the surface. The millimeter wave thermographic units are designed to contribute to the clinical detection of breast abnormalites with the specific aim of accurately and noninvasively detecting breast cancer.

A new computerized fluoroscopy technique for isolating low imagecontrast, which results during the flow of an intravenously administered bolus of iodinated contrast materials, is proposed. The technique requires the application of one of a family of imaging algorithms which have been designed to isolate time varying imagecontrast. This family of imaging algorithm is described, as is the way in which each isolates a particular range of temporal frequency components associated with the bolus’s flow through various cardiovascular structures. The implementation of these algorithms in real time using appropriate digital recursive filtering techniques is described. The architecture of a dedicated hardwired computerized fluoroscopy apparatus which would incorporate these algorithms is also presented.

Our lab has previously generated selective iodine images with an image intensifier fluoroscopic system using a three‐beam K‐edge approach. Logarithmically amplified video images Li were linearly combined to yield the final image k1L1+k2L2+k3L3. This paper discusses refinements of the K‐edge technique. A study is made of the manner in which contrast‐reducing effects such as x‐ray scatter and image intensifier veiling glare enter into the final image. If such biases can be approximated as multiplicative and independent of the x‐ray spectrum, and if the sum of the ki is zero, then the biases are canceled. Experimental data is presented which demonstrates that the inaccuracy due to such biases can be reduced by a factor as large as 10. The theorem that k1+k2+k3≊0 is proven rigorously and discussed. Because the ki add to zero, the final image can be expressed as a linear combination of two of the differences between the Li. A difference‐based algorithm which reduces biases and make allowance for nonlinearities such as beam hardening is proposed and discussed.

In an earlier article we discussed the rationale for using differences between video images in three‐beam selective iodine K‐edge imaging. Rather than combining three initial imagesLi linearly to yield the final imagek1L1+k2L2+k3L3, differences between the Li were first generated and then combined either to linear or quadratic order. This approach was motivated by the desire to suppress the large multiplicative biases of fluoroscopic imaging and justified by theoretically proving that k1+k2+k3≊0. In this paper we discuss the instrumentation and experimental results obtained from this difference‐based technique. A specially‐constructed apparatus is described which automatically selects the optimum combination coefficients and combines the difference images up to quadratic order at realtime video rates. Three methods for generating K‐edge subtraction images are compared: the former approach in which the Li are linearly combined and combination of differences to linear and quadratic order. In imaging phantoms in which the iodine distribution is known, the resultant subtraction images from all three methods appear similar. Inspection of signal sizes shows that the quadratic difference‐based approach provides superior bone and tissue residual suppression by about a factor of 2. In imaging phantoms in which the iodine distribution is unknown, incomplete suppression of x‐ray scatter and image intensifier veiling glare prevent a quantitative comparison of performance of the three algorithms. An experiment verification is provided of the theorem which states that k1+k2+k3≊0.

A method for describing the absorbed dose delivered by x‐ray transmission computed tomography(CT) is proposed which provides a means to characterize the doses resulting from CT procedures consisting of a series of adjacent scans. The dose descriptor chosen is the average dose at several locations in the imaged volume of the central scan of the series. It is shown that this average dose, as defined, for locations in the central scan of the series can be obtained from the integral of the dose profile perpendicular to the scan plane at these same locations for a single scan. This method for estimating the average dose from a CT procedure has been evaluated as a function of the number of scans in the multiple scan procedure and location in the dosimetry phantom using single scan dose profiles obtained from five different types of CT systems. For the higher dose regions in the phantoms, the multiple scan dose descriptor derived from the single scan dose profiles overestimates the multiple scan average dose by no more than 10%, provided the procedure consists of at least eight scans.

The theory developed by Muntz describing the relative detectability of calcifications in mammography has been recast in a way that permits meaningful experimental verification. An important result of this study is the demonstration of the power of using a normalization to a reference condition. Properly employed, normalization suppresses many of the difficult physical and psychovisual factors that appear in any attempt at analyzing radiographic examinations.

Spectral quality at different locations across a 40×40 cm2 field and on the central‐axis for different field sizes from Clinac‐4 and Clinac‐6 accelerators has been measured using the photoactivation ratio method. This method is based on a measurement of the ratio of photoactivation rates induced by 115In(γ,γ′) and D(γ,n)×115In(n,γ). Despite the high sensitivity of this technique, no significant variation of spectral quality across the irradiation field in air and for different field sizes on the central‐axis is observed.

Many radiotherapylinear accelerators use electron beam applicators which extend close to the patient’s surface when treating at the regular distance. The relatively large size of these applicators often necessitates the use of a larger SSD than that designed by the manufacturer. In such cases, the addition of shielding blocks to the applicator can significantly alter the factors which should be used to calculate the dose rate at the new SSD as compared with the unshielded beam condition. In some typical clinical situations, errors of greater than 60% may result from failure to account for this perturbation. This paper presents the proper correction of the dose vs SSD function for the presence of such shielding blocks for the Clinac‐18 linear accelerator.

The surface radio frequency (rf) power absorption in human head and torso nuclear magnetic resonance(NMR)imaging experiments is estimated. The results are expressed as a function of the NMR frequency, the rf pulse length, and the pulse duty cycle, which are varied over six orders of magnitude for general applicability. The results are compared with average metabolic levels and the limits advised by the National Radiological Protection Broad of the United Kingdom. Heating due to time‐dependent magnetic field gradients is discussed.

It has recently been suggested that values for the speed of sound in tissues currently reported in the literature are higher than those presently in use in diagnosticultrasound. The purpose of this note is to indicate that the newer data under criticism was obtained at body temperature (and higher), whereas speed of sound in tissue data currently in use by medical physicists appear to be based on measurements made at room temperature. If a positive ultrasoundtemperature coefficient for soft tissue is used to extrapolate the data from body to room temperature, it can be seen that the extrapolation falls within the region of the bulk of room temperature measurements. Fortunately, most applications in which speed of sound data in tissue is used involves partial paths through lipidous tissue. Since the speed of sound in lipidous tissue is relatively lower at body temperature(lipid has a negative temperature coefficient) than is the speed of sound in soft tissue, the mean speed is lower than that obtained in soft tissue alone and room temperature values are probably justified. However, medical physicists should be careful to use body temperature data for soft tissue when the insonified region is composed mainly of soft tissue.

One of the important characteristics of a computed tomographyscanner is the image slice thickness. Most phantoms designed to measure this parameter do so with a ramp or tilted wire. Such a phantom must be precisely aligned to avoid possible significant inaccuracy. We present here a procedure for measuring the image slice thickness using a phantom containing two crossed ramps. The procedure produced consistent and accurate measurements of slice thickness without having to carry out a time consuming alignment procedure.

A general method for calculating the dose distribution in an irradiated volume is to evaluate the primary and scatter components separately according to the method described by Clarkson and Cunningham. It was found, however, that for a 6 MV Siemens accelerator the calculated dose overestimated the peripheral dose at depths beyond 10 cm by 3%–6%. The difference was attributed to the varying beam quality across the field. This beam quality variation was decreased by hardening the beam with a permanently installed 1/8 in. tungsten filter inserted between the beam flattening filter and the mirror base assembly. The tungsten filter had a more pronounced beam hardening effect at the beam edge than along the central ray. For example, the dose rate in air at the beam edge for a 30×30 cm2 field was 13% higher than along the central ray without the tungsten filter. The addition of the 1/8 in. filter decreased this horn to 6%. The beam quality along the central ray also increased. The tissue–air ratio for zero field size along the central ray increased by 2% with the addition of the tungsten filter. The scatter–air ratio, however, did not change with the added filter. Agreement within 1%–2% was achieved between the calculated and measured beam profiles at all depths in a phantom when the tungsten filter was added.